Film transport mechanisms used in photographic film scanners can be configured with DC or AC motors, stepper motors or servo motors used to operate the transport mechanism at different film transport speeds. Film scanners require precise positioning of the film strip, particularly during the image scanning operation for quality results. Drives for this purpose typically include DC motors with position feedback using encoder wheels or disks. Transport speed can be changed by varying the voltage supplied to the DC motor thus making the selection of appropriate operating speed a relatively simple matter. However, the requirement for precise positioning makes the feedback control with encoder wheels a complex and relatively costly approach to transport design. Stepper motor drive systems have the advantage that precise positioning can be readily achieved. However, a film scanner having a high/low speed ratio of, for example, on the order of 40 to 1, requires that a relatively costly stepper motor be used to realize such a high range of speed variation since inexpensive stepper motors are generally limited to a low range of speed variation. For film scanners intended to be mass produced for consumer applications, it is necessary to minimize the product cost which mitigates against use of the more costly stepper motor transport designs.
In the above cross referenced U.S. Application, a pivoted gear module is employed to selectively engage a high speed or low speed intermediate gear drive between an input drive gear attached to a low cost, limited range stepper motor, and an output driven gear attached to a film drive apparatus. The gear module is pivoted by means of a cam disk driven by a reversible DC motor. When shifting gears from one intermediate drive to another, the input load should be applied only when the teeth are in adequate mesh to prevent tooth damage. This requires that the gears are set to the correct center distance to ensure that the contact ratio is greater than or equal to one, meaning that there is always at least one tooth of the driving gear in contact at the involute surface with one tooth of the driven gear at its involute surface. Alignment of the input drive gear and the output driven gear teeth to the teeth of the intermediate gears is important in order to swing the intermediate gear module fully into position. If the teeth on the input drive gear are oriented in such a way to the intermediate gear drive teeth that either a tip to tip contact condition or two teeth on the input drive gear straddling two teeth on the intermediate gear drive results, then the gear teeth will not be fully engaged. The same situation can occur at the intermediate gear drive to output driven gear interface. These interference conditions are referred to herein as "stubbing" of the gear teeth. If a stubbing condition at either or both of the gear interfaces during the gear meshing procedure, then the intermediate gear drive will not fully engage with the input drive gear and output driven gear and the drive torque will not be properly transmitted to the driven output gear. It is known to use active sensing and feedback mechanisms to detect the occurrence of stubbing and provide feedback to initiate corrective actions to clear the stub condition. However, these mechanisms can be complex and require added components and cost to the gear shift mechanism. There is therefore a need for an arrangement that ensures proper meshing of intermediate gear drives to input drive and output driven gears in a dual speed gear shift mechanism particularly for a film drive apparatus that does not require active feedback mechanisms to detect and clear gear stub conditions and that maintains accurate relational position of the drive motor setting to the film drive positioning. The present invention satisfies that need.